Grouping numbers to add
Understand and apply the associative property of addition: when adding three numbers, any two can be added first
What a learner can do afterwards
- Solve 2 + 6 + 4 by first adding 6 + 4 = 10, then 2 + 10 = 12
- Explain that grouping addends differently gives the same total
- Choose which two numbers to add first to make the calculation easier
The lesson
You already know you can add two numbers in any order. Grouping works in a similar way. When you add three numbers, you can add any two of them first, then add the third one. The total stays the same either way.
Ben adds 3 + 7 + 6. He notices that 3 and 7 make 10, an easy number. So he groups those first: 3 + 7 = 10. Then he adds 10 + 6 = 16. Grouping the numbers a different way did not change the answer.
Look for two numbers that make a friendly number like 10. Add those first, then add the last number. The total never changes, no matter which pair you pick first.
You can add any two numbers first when adding three of them, and the total always stays the same.
Watch it
Where it sits
This opens up
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.